Design and preparation of CdS/H-3D-TiO2/Pt-wire photocatalysis system with enhanced visible-light driven H2 evolution

In recent years, tremendous efforts have been devoted to develop new photocatalyst with wide spectrum response for H₂ generation from water or aqueous solution. In this work, CdS nanoparticles (NPs) have been immobilized on hydrogenated three-dimensional (3D) branched TiO₂ nanorod arrays, resulting in a highly efficient photocatalyst, i.e, CdS/H-3D-TiO₂. In addition, electrochemical reduction of H⁺ ion is identified as a limiting step in the photocatalytic generation of H₂ at this catalyst, while here a Pt wired photocatalysis system (CdS/H-3D-TiO₂/Pt-wire) is designed to overcome this barrier. Without the application of potential bias, visible light photocatalytic hydrogen production rate of CdS/H-3D-TiO₂/Pt-wire is 18.42 μmol cm^?2 h^?1, which is 11.2 times that of CdS/H-3D-TiO₂ without Pt (1.64 μmol cm^?2 h^?1). The Pt wire acts as an electron super highway between the FTO substrate and H⁺ ions to evacuate the generated electrons to H⁺ ions and catalyze the reduction reaction and consequently generate H₂ gas. This work successfully offers a novel direction for dramatic improvement in H₂ generation efficiency in photocatalysis field.     

Investigation of the interaction between intermediates from gasification of biomass in supercritical water: Formaldehyde/formic acid mixtures

Supercritical water gasification (SCWG) is a promising technology for converting wet biomass and waste into renewable energy. While the fundamental mechanism involved in SCWG of biomass is not completely understood, especially hydrogen (H₂) production produced from the interaction among key intermediates. In the present study, formaldehyde mixed with formic acid as model intermediates were tested in a batch reactor at 400 °C and 25 MPa for 30 min. The gas and liquid phases were collected and analyzed to determine a possible mechanism for H₂ production. Results clearly showed that both gasification efficiency (GE) and hydrogen efficiency (HE) increased with addition of formic acid, and the maximum H₂ yield reached 17.92 mol/kg with a relative formic acid content of 66.67% in the mixtures. The total organic carbon removal rate and formaldehyde conversion rate also increased to 67.33% and 89.81% respectively. The reaction pathways for H₂ formation form mixtures was proposed and evaluated, formic acid promoted self-decomposition of formaldehyde to generate H₂, and induced a radical reaction of generated methanol to produce more H₂.     

Manifold improvement of water oxidation activity of NaCoO2 by selective cation exchange

Water oxidation produces electrons, protons, and oxygen, essential components in fuel production processes like CO₂ reduction and hydrogen generation. To date, less cost and more-abundant catalysts showed low activity for the water oxidation reaction. Herein, we developed a simple and effective way to selectively exchange Na⁺ with H⁺ in NaCoO₂ and created high surface area layered materials for efficient water oxidation. The photochemical water oxidation reaction activity improved from 0.9 × 10^?3 s^?1 (pristine NCO) to 2.90 × 10^?3 s^?1 (AT-NCO-2M). The electrochemical overpotential decreased from 600 to 370 mV, and the Tafel slope dropped from 93 to 47 mV/dec. Enhanced water oxidation activity is owing to delamination of stacked layered structure into few layer structure, increased surface area, super hydrophilic nature, and coexistence of Co⁴⁺ and Co³⁺ species. Our findings show the importance of the exchange of Na⁺ with H⁺ in Na-based metal oxides for efficient water oxidation.     

Supercritical water gasification of food waste: Effect of parameters on hydrogen production

Food waste is a type of municipal solid waste with abundant organic matter. Hydrogen contains high energy and can be produced by supercritical water gasification (SCWG) of organic waste. In this study, food waste was gasified at various reaction times (20–60 min) and temperatures (400 °C-450 °C) and with different food additives (NaOH, NaHCO₃, and NaCl) to investigate the effects of these factors on syngas yield and composition. The results showed that the increase in gasification temperature and time improved gasification efficiency. Also, the addition of food additives with Na⁺ promoted the SCWG of food waste. The highest H₂ yield obtained through non-catalytic experiments was 2.0 mol/kg, and the total gas yield was 7.89 mol/kg. NaOH demonstrated the best catalytic performance in SCWG of food waste, and the highest hydrogen production was 12.73 mol/kg. The results propose that supercritical water gasification could be a proficient technology for food waste to generate hydrogen-rich gas products.     

Influence of AlCl3 and oxidant catalysts on hydrogen production from the supercritical water gasification of dewatered sewage sludge and model compounds

Hydrogen has attracted significant attention as a clean energy source. Supercritical water gasification (SCWG) technology can produce hydrogen-rich gas while also disposing of sludge. The hydrogen yield from the SCWG of sludge is greatly increased when catalyzed by AlCl₃. In this paper, a combined catalyst based on AlCl₃ was proposed to further increase the hydrogen yield of SCWG of dewatered sewage sludge (DSS). Analysis of the products from catalytic gasified of DSS and its model compounds were used to propose a catalytic mechanism and reaction pathway of the catalytic SCWG of DSS. Among the combined catalysts used for the SCWG of DSS, 10 wt% AlCl₃–H₂O₂ (mass ratio 8:2) had the best hydrogen production effect, and the hydrogen yield reached 8.88 mol/kg organic matter. This was 14% higher than when catalyzed by 10 wt% AlCl₃. During catalysis with AlCl₃, Al³⁺ reacted with OH⁻ in water and precipitated as Al(OH)₃, which produced an acidic environment in the liquid product. Al(OH)₃ dehydrated to form an AlO(OH) and deposited in the solid product. A small amount of H₂O₂ promoted the steam reforming reaction of organic matter in DSS, which increased the hydrogen yield. H₂O₂ further promoted the hydrogen yield in an acidic environment. The catalytic effect of AlCl₃ was unaffected by H₂O₂. The H⁺ generated by AlCl₃ during catalysis promoted H₂O₂ to further depolymerized organic matter (such as humic substances) in DSS, so that AlCl₃–H₂O₂ catalyzed the SCWG of DSS to further increase the hydrogen yield. The order of hydrogen yield catalyzed by AlCl₃–H₂O₂ was guaiacol > humic acid > glycerol > alanine > glucose. Compared with AlCl₃, AlCl₃–H₂O₂ reduced the hydrogen yield of glucose by nearly 20% and increased the hydrogen yield of humic acid by about 17% (25.81 mol/kg feed).     

Supercritical water synthesized Ni/ZrO2 catalyst for hydrogen production from supercritical water gasification of glycerol

Catalysts are crucial to promote the technical feasibility of supercritical water gasification (SCWG) for H₂ production from wet biomass, yet catalysts prepared by conventional methods normally encounter sintering problems in supercritical water. Herein, a series of ZrO₂-supported Ni catalysts were tried to be prepared by supercritical water synthesis (SCWS) and evaluated for SCWG in terms of activity and property stability. The SCWS was conducted at 500 °C and 23 MPa using metal nitrates as starting materials. Effect of precursor concentration on property and catalytic performance of the SCWS-prepared catalysts for SCWG of 20 wt% glycerol were systematically studied. XRD, SEM-EDS, TEM and TGA were applied for catalyst characterization. Results verified the successful obtaining of Ni/ZrO₂ nanocatalysts with Ni crystals of 30–70 nm and ZrO₂ crystals of ~11 nm by the SCWS process, which were found to be active on the WGSR for SCWG to increase the H₂ yield as high as 155%. Importantly, the SCWS-prepared Ni/ZrO₂ catalysts exhibited excellent property stability and anti-coking ability for SCWG of glycerol.     

585 divacancy-defective germanene as a hydrogen separation membrane: A DFT study

As sustainable and clean energy, hydrogen is the most attractive and promising energy source in the future. Membrane separation is attractive due to its high hydrogen separation performance and low energy consumption. Van-der-Waals-corrected density functional theory (DFT) calculations are performed to investigate the hydrogen separation performance of 585 divacancy-defective germanene (585 germanene). It is found that the 585 germanene presents a surmountable energy barrier (0.34 eV) for hydrogen molecule passing through the membrane, and that membrane exhibits extremely high selectivity for H₂ molecules over CO, CO₂, N₂, CH₄ and H₂S molecules in a wide range of temperatures. Meanwhile, the hydrogen permeance of 585 germanene can reach 1.94 × 10^?7 mol s^?1 m^?2 Pa^?1 at the low limit temperature of methane reforming (at 450 K), which is higher than the industrially acceptable gas permeance. With high selectivity and permeance, the 585 germanene is a promising candidate for hydrogen separation.     

A coordinatively saturated cobalt complex as a new kind catalyst for efficient electro- and photo-catalytic hydrogen production in purely aqueous media

It has been shown that coordinatively unsaturated complexes can catalyze hydrogen production via an unstable hydride intermediate. Herein we present a new kind of water soluble catalyst based on a coordinatively saturated cobalt complex, [(phen)₂Co(CN)₂]?ClO₄1 that is formed by the reaction of 1,10-phenanthroline (phen), Co(ClO₄)₂·6H₂O and tetracyanoethylene (TCNE). Under photoirradiation with blue light (λ_max = 469 nm) in air, together with [Ru(bpy)₃]Cl₂ and ascorbic acid in a pH 5.5 aqueous solution, 1 possesses photocatalytic activity for water reduction to hydrogen with an initial turnover number (TON) of 1232H₂ per mol of catalyst at first 10 h, and this activity is sustained for at least 70 h. This can be attributed to that oxidative quenching by 1 (k_q = 1.69 × 10¹⁰ M^?1 s^?1) dominates over reductive quenching to [Ru(bpy)₃]Cl₂ by ascorbic acid (k_q = 1.55 × 10¹⁰ M^?1 s^?1). Additionally, 1 electrocatalyze hydrogen generation from a neutral water with a turnover frequency (TOF) of 1113.1 mol of hydrogen per mole of catalyst per hour (mol H₂/mol catalysts/h) at an overpotential (OP) of 838 mV. We hope this can afford a new method in proton or water reduction catalysis using coordinatively saturated complexes in purely aqueous media.     

Thermodynamic study on the integrated supercritical water gasification with reforming process for hydrogen production: Effects of operating parameters

In this paper, a conceptual process design of the integrated supercritical water gasification (SCWG) and reforming process for enhancing H₂ production has been developed. The influence of several operating parameters including SCWG temperature, SCWG pressure, reforming temperature, reforming pressure and feed concentration on the syngas composition and process efficiency was investigated. In addition, the thermodynamic equilibrium calculations have been carried out based on Gibbs free energy minimization by using Aspen Plus. The results showed that the higher H₂ production could be obtained at higher SCWG temperature, the H₂ concentration increased from 5.40% at 400 °C to 38.95% at 600 °C. The lower feed concentration was found to be favorable for achieving hydrogen-rich gas. However, pressure of SCWG had insignificant effect on the syngas composition. The addition of reformer to the SCWG system enhanced H₂ yield by converting high methane content in the syngas into H₂. The modified SCWG enhanced the productivity of syngas to 151.12 kg/100kg_feed compared to 120.61 kg/100kg_feed of the conventional SCWG system. Furthermore, H₂ yield and system efficiency increased significantly from 1.81 kg/100kg_feed and 9.18% to 8.91 kg/100kg_feed, and 45.09%, respectively, after the modification.     

Economic evaluation with sensitivity and profitability analysis for hydrogen production from water electrolysis in Korea

Economic evaluation for water electrolysis compared to steam methane reforming has been carried out in terms of unit hydrogen production cost analysis, sensitivity analysis, and profitability analysis to assess current status of water electrolysis in Korea. For a hydrogen production capacity of 30 Nm³ h^?1, the unit hydrogen production cost was 17.99, 16.54, and 20.18 $ kg H₂^?1 for alkaline water electrolysis (AWE), PEM water electrolysis (PWE), and steam methane reforming (SMR), respectively with 11.24, 10.66, and 11.80 for 100 Nm³ h^?1 and 8.12, 7.72, and 7.59 $ kg H₂^?1 for 300 Nm³ h^?1. With sensitivity analysis (SA), the most influential factors on the unit hydrogen production cost depending on the hydrogen production capacity were determined. Lastly, profitability analysis (PA) presented a discounted payback period (DPBP), net present value (NPV), and present value ratio (PVR) for a different discount rate ranging from 2 to 14% and it was found that a discounted cash flow rate of return (DCFROR) was 14.01% from a cash flow diagram obtained for a hydrogen production capacity of 30 Nm³ h^?1.     

Graphite coating on alumina substrate for the fabrication of hydrogen selective membranes

Development of composite membranes is a suitable alternative to improve the hydrogen flux through palladium membranes. The porous substrate should not represent a barrier to gas permeation, but the roughness of its surface should be sufficiently smooth for the deposition of a thin and defect-free metal layer. In this study, the performances of the modification of the outer surface of an asymmetric alumina hollow fibre substrate by the deposition of a graphite layer were evaluated. The roughness of the substrate outer surface was reduced from 120 to 37 nm after graphite coating. After graphite coating, the hydrogen permeance through the composite membrane produced with 2 Pd plating cycles was of 1.02 × 10^?3 mol s^?1 m^?2 kPa^?1 at 450 °C and with infinite H₂/N₂ selectivity. Similar hydrogen permeance was obtained with the composite membrane without graphite coating, also at infinite H₂/N₂ selectivity, but 3 Pd plating cycles were necessary. Thus, graphite coating on asymmetric alumina hollow fibres is a suitable alternative to reduce the required palladium amount to produce hydrogen selective membranes.     

Phosphorus-doped nickel sulfides/nickel foam as electrode materials for electrocatalytic water splitting

Hydrogen production from electrocatalytic water splitting is viewed as one of the most promising methods to generate the clean energy. In this work, we successfully prepared an electrode material by growing phosphorus-doped Ni₃S₂ (PNi₃S₂) on nickel foam substrate (NF) under hydrothermal conditions. The phosphorus-doping has an obvious effect on the morphology of Ni₃S₂ on the surface of the nickel foam, which probably results in more active sites, higher electrical conductivity and faster mass transfer. The resulting electrode material displays excellent electrocatalytic activities and stability towards both OER (oxygen evolution reaction) and HER (hydrogen evolution reaction). A relatively low overpotential of 306 mV is required to reach the current density of 100 mA cm^?2 for OER and 137 mV at 10 mA cm^?2 for HER in 1 M KOH solution. When PNi₃S₂/NF was used in an electrolyzer for full water splitting, it can generate a current density of 10 mA cm^?2 at 1.47 V with excellent stability for more than 20 h.     

Storing-hydrogen processes on graphene activated by atomic-vacancies

We investigated the minimum energy pathways and energy barriers of reversible reaction (V₁₁₁ + H₂?V₂₂₁) based upon calculations using density functional theory. We find a comparable activation barrier of around 1.3 eV for both the dissociative chemisorption and desorption processes. The charge transfer rate from a reacting hydrogen atom to the graphene is around 0.18 e per hydrogen atom in the final state. A subsequent reaction path to recover the initial structure of V₁₁₁ is realized by the migration of hydrogen atoms from V₂₂₁ onto the graphene surface. The comparable energy barrier of 1.3 eV for both adsorption and desorption suggests that this novel storage and release concept has the potential to act as a hydrogen storage system for certain applications.     

A new hydrogenation-coupling approach for supra-equilibrium conversion in a water–gas shift reaction: Simultaneous hydrogen generation and chemical storage

This work reports a practical system of hydrogenation-coupled water–gas shift reaction (HC-WGSR) for simultaneous hydrogen production and storage. The performance of the HC-WGSR system was predicted through thermodynamic simulation. The proof-of-concept tandem water–gas shift and propene hydrogenation strategy was successfully demonstrated using a bifunctional catalyst. The hydrogen produced from the WGSR was successfully stored in propane simultaneously, and the overall CO conversion of nearly 100% overcame the equilibrium limitation of the WGSR over a wide range of space velocities (3000 - 6000 h⁻¹) at 200 ^°C and 1 bar. This study demonstrated that the in situ removal/storage of H₂ using the hydrogenation-coupling approach is promising even in a CO₂-rich environment (20% CO₂). The new approach shall see a great opportunity in using organic hydrogen carriers, e.g., benzene, toluene, N-ethylcarbazole, to expand the industrial applications, underpinning the global supply chain for hydrogen energy.     

Reversible H2/H2O electrochemical conversion mechanisms on the patterned nickel electrodes

The patterned nickel (Ni) electrode enables to quantify the triple-phase boundary (TPB) length and Ni surface area as well as exclude the interference of bulk gas diffusion. In this study, the patterned Ni electrodes are investigated in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes at the atmosphere of H₂O/H₂. The experimental test shows the patterned Ni electrode keeps stable and intact only at the specific operating condition due to instability of Ni at the H₂O-containing atmosphere. The effects of the temperature, partial pressure of H₂O and H₂ on the electrochemical performance are measured. The electrochemical performance has a positive correlation with the temperature, partial pressure of H₂ and H₂O. Further, the experimental results are compared with the mechanism containing two-step charge-transfer reaction used in the existing literature. An analytical calculation is performed to indicate the rate-limiting steps may be different for SOFC and SOEC modes. In SOFC mode, H₂ electrochemical oxidation could be dominated by both charge transfer reaction at low polarization voltage and by the charge-transfer reaction H(Ni) + O^2?(YSZ) → OH^?(YSZ) + (Ni) + e^? at high polarization voltage, however in SOEC mode, H₂O electrochemical reduction is considered to be dominated by H₂O(YSZ) + (Ni) + e^? → OH^?(YSZ) + H(Ni).     

Fabrication of nanoporous Si electrocathode by high-energy argon ion irradiation for improved electrocatalytic hydrogen production

Developing a cost-effective material to replace Pt catalysts for hydrogen evolution reaction (HER) holds great promising for clean energy technologies. In this work, we developed a simple way to prepare nanoporous Si by argon ion irradiation at 90 keV to fluences of 0.5, 1, 2 × 10¹⁷ ions/cm². After post-irradiation annealing at 700 °C in vacuum (2 × 10^?4 pa) for 5 h, the nanoporous Si was formed which displays largely enhanced electrocatalytic water splitting compared with the unirradiated Si. We also investigated the influence of fluence on morphology and the HER performance of the electrocathodes. It was found that 1 × 10¹⁷ Ar⁺ ions/cm² irradiated Si showed the highest HER performance. The largely enhanced HER activity comes from the unique morphology which results by Ar⁺ ions irradiation and post-irradiation annealing. We further fabricated electrocathodes by coating Ni film on the nanoporous Si, significant improvement of the HER performance compared with Ni coated planar Si was found. Using the ion irradiation technology, we developed a new method to fabricate electrocathode with large specific surface area for largely enhanced hydrogen evolution reaction activities.     

Hydrogen generation by aluminum‐water reaction in acidic and alkaline media and its reaction dynamics

The additives AlCl₃, CoCl₂, Al(OH)₃, Ca(OH)₂, and NaAlO₂ are added to water to regulate its pH value (pH = 2‐13) in this study. The effects of media on the aluminum‐water reaction are investigated. Up to an increase in temperature, the hydrogen generation rate in different media increases. H⁺, OH^?, Cl^?, or Co produced from the additive favors the initial removal of the oxide film and aluminum corrosion. Therefore, the initial hydrogen generation rate increases in acidic and alkaline media. The synergistic effect of the formed fresh Co and Cl^? catalyzes aluminum‐water reactions. However, the amount of hydrogen decreases with increasing mass of CoCl₂ because of agglomeration of the catalyst Co. The higher concentration of OH^? ions aids hydrogen generation. However, the reaction rate became slow after the rapid consumption of OH^?, when the concentration of OH^? was relatively small. Hydrogen is quickly generated and Al is completely reacted upon following additions of Al due to the cooperation between H⁺, Cl^?, OH^? ions, and the formed Al(OH)₃.     

Co Nanoparticles@N-doped carbon coated on carbon Nanotube@Defective silica as non-noble photocathode for efficient photoelectrochemical hydrogen generation

The development of photoelectrodes capable of light-driven hydrogen evolution from water with non-noble metals is an important approach for the storage of solar energy in the form of a chemical energy carrier. In this study, we report Co nanoparticles@N-doped carbon coated on carbon nanotube@defective-silica (CNTs@Co@NC/D-SiO₂), which are composed of Co nanoparticles@N-doped carbon as electrocatalyst, defective-silica as photocatalyst and carbon nanotube as conductive substrates. The obtained non-noble photocathode possesses the high performance for efficient photoelectrochemical hydrogen evolution reaction. When evaluated for hydrogen evolution reaction electrocatalysis, CNTs@Co@NC/D-SiO₂ exhibits a small onset overpotential of 104 mV (J = 1 mA cm^?2), a Tafel slope of 69.1 mV dec^?1 and outstanding long-term cycling stability. The P type semiconductor characteristics of CNTs@Co@NC/D-SiO₂ due to defective-silica with carrier concentration of 3.53 × 10¹⁹ cm^?3 is measured, which produces a significant positive shift of overpotential of 40 mV (J = 10 mA cm^?2) under 100 mW cm^?2 simulated sunlight irradiation. These findings provide a straightforward and effective route to produce cheap and efficient photo-electro-catalyst for water splitting.     

Experimental investigation of solar assisted hydrogen production from water and aluminum

Sustainable production of hydrogen at high capacities and low costs is one the main challenges of hydrogen as a future alternative fuel. In this paper, a new hydrogen production system is designed and fabricated to investigate hydrogen production using aluminum and solar energy. Numerous experiments are performed to evaluate the hydrogen production rate, quantitatively and qualitatively. Moreover, correlations between the total hydrogen production volume over time and other parameters are developed and the energy efficiency and conversion ratio of the system are determined. Also, a method is developed to obtain an optimal and stable hydrogen production rate based on system scale and consumed materials. It is observed that at low temperatures, the hydrogen production volume, efficiency and COP of the system increase at a higher sodium hydroxide molarity. In contrast, at high temperatures the results are vice versa. The maximum hydrogen production volume, hydrogen production rate, reactor COP and system efficiency using 0.5 M NaOH solution containing 3.33 g lit^?1 aluminum at 30 °C are 6119 mL, 420 mL min^?1, 1261 mL H₂ per 1 g of Al, and 16%, respectively.     

Thermodynamic analysis of hydrogen production from methane via autothermal reforming and partial oxidation followed by water gas shift reaction

Reaction characteristics of hydrogen production from a one-stage reaction and a two-stage reaction are studied and compared with each other in the present study, by means of thermodynamic analyses. In the one-stage reaction, the autothermal reforming (ATR) of methane is considered. In the two-stage reaction, it is featured by the partial oxidation of methane (POM) followed by a water gas shift reaction (WGSR) where the temperatures of POM and WGSR are individually controlled. The results indicate that the reaction temperature of ATR plays an important role in determining H₂ yield. Meanwhile, the conditions of higher steam/methane (S/C) ratio and lower oxygen/methane (O/C) ratio in association with a higher reaction temperature have a trend to increase H₂ yield. When O/C ≤ 0.125, the coking behavior may be exhibited. In regard to the two-stage reaction, it is found that the methane conversion is always high in POM, regardless of what the reaction temperature is. When the O/C ratio is smaller than 0.5, H₂ is generated from the partial oxidation and thermal decomposition of methane, causing solid carbon deposition. Following the performance of WGSR, it suggests that the H₂ yield of the two-stage reaction is significantly affected by the reaction temperature of WGSR. This reflects that the temperature of WGSR is the key factor in producing H₂. When methane, oxygen and steam are in the stoichiometric ratio (i.e. 1:0.5:1), the maximum H₂ yield from ATR is 2.25 which occurs at 800 °C. In contrast, the maximum H₂ yield of the two-stage reaction is 2.89 with the WGSR temperature of 200 °C. Accordingly, it reveals that the two-stage reaction is a recommended fuel processing method for hydrogen production because of its higher H₂ yield and flexible operation.